The present invention relates to a heat-sensitive recording material, particularly to a heat-sensitive recording material that can retain its excellent recording ability for a long period of time and can produce recorded matter having excellent stability. More specifically, the present invention relates to a heat-sensitive recording material which is excellent in color-forming properties upon application of heat, and which can produce recorded matter in which the whiteness of an unprinted area is high; that is, a printed image of high contrast.
Heat-sensitive recording methods typically utilize a recording material as prepared by mixing a basic dye precursor, an acidic substance and a low melting compound in a particle form as the color-forming components and coating the resulting mixture on a support, in which upon application of heat the particles melt, thereby forming color. Such recording materials, however, have a disadvantage in that when it is handled under severe conditions after recording or contacted with an adhesive tape or a diazo copying paper, decoloration or coloration undesirably occurs.
In recent years, to overcome the above problem, an improved heat-sensitive recording material has been developed in which one of the color-forming components is encapsulated.
U.S. Pat. No. 4,529,681, for example, discloses a light-sensitive, heat-sensitive recording material in which microcapsules containing a photopolymerizable vinyl compound, a photopolymerization initiator, and one of the components causing color formation, and the other color-forming component are coated on the same surface of a support. When the above recording material is heated, the color-forming component in the capsule permeates through the capsule wall, coming out of the capsule, or alternatively the other color-forming component outside the capsule permeates through the capsule wall, entering the inside of the capsule. This results in the formation of color. That is, upon application of heat, color is formed in heated areas. Thereafter, if the material is entirely exposed to light, the vinyl compound in the core of the capsule undergoes polymerization, thereby preventing the permeation of the color-forming compound through the capsule wall and thus preventing coloration of non-colored areas. This operation to prevent further coloration is usually called "fixing".
U.S. patent application Ser. No. 600,267 (filed on Apr. 13, 1984) discloses a material in which at least one of a diazo compound, a coupling component and an auxiliary color-forming agent is incorporated in the core of a microcapsules.
The above photo-fixable, heat-sensitive recording material utilizing microcapsules has advantages in that a recording apparatus can be simplified, the material can be stored for a long period of time while retaining its recording performance, and in that the stability of an image and its background after recording is excellent. However, on the other hand, it has disadvantages in that since at least one of color-forming components causing color formation is separated by the capsule walls, heat color-forming properties are reduced and at high-speed recording utilizing pulses of shorter width, coloration sometimes occurs only to an insufficient extent.
In an embodiment of U.S. Pat. No. 4,529,681, when a basic colorless dye is used as a color-forming component, it is essential for the composition of a core substance to have such properties as to undergo photopolymerization upon application of light and to harden. Even in the case of a core substance composition not having a photopolymerization capability, that is, a composition not containing a vinyl compound and photopolymerization initiator at the same time, the material is excellent in storage stability and also in the stability of recorded matter, such as a printed image, but the heat color-forming properties thereof are reduced, since the color-forming components are separated by the capsule walls before and after recording. In a case that a vinyl compound coexists, coloration of a basic dye precursor at the step of encapsulation does not almost occur. In the system of U.S. Pat. No. 4,529,681, however, if a heat-sensitive recording material not having fixing properties is intended to prepare without use of a vinyl compound, a problem arises in that coloration readily occurs at the step of encapsulation. If such colored microcapsules are used, a sheet prepared by coating the microcapsules has a disadvantage in that the whiteness is low.